This invention relates to the manufacture of foundry moulds and cores which do not evolve pungent acid gases on thermal decomposition. More particularly it refers to a method of making moulds and cores of this type rapidly at ambient temperature.
Phenol formaldehyde and phenol formaldehyde/furfuryl alcohol condensation products catalyzed with strong acids such as sulphuric acid, paratoluene sulphonic acid, are well known as binders for sand in the production of foundry moulds and cores. However, they have the disadvantage that pungent fumes of sulphur dioxide are evolved on thermal decomposition.
The use of alkaline phenolic resins catalyzed with esters has been suggested in Japanese Patent Publication No. 130627/1975 and is the subject of co-pending U.S. Application Ser. No. 224,131, filed Jan. 12, 1981, now U.S. Pat. No. 4,426,467, issued Jan. 17, 1984 and U.S. Ser. No. 434,462 filed Oct. 14, 1982, now abandoned. The use of such binder systems enables the manufacture of foundry moulds and cores which do not evolve pungent acid gases on mixing or during casting. Further, by suitable selection of resins and ester catalysts rapid hardening at ambient temperature can be achieved. However, to obtain such results on a large scale it is necessary to use specialized rapid mixing equipment such as that described in British patent specification Nos. 1257181 and 1369445 of Baker Perkins.
The present invention is based on the discovery that the use of esters as catalysts for alkaline phenolic resins in the manufacture of foundry moulds and cores can be adapted to a gassing system which is capable of rapid cure at ambient temperature. The use of gassing to promote curing of binders for foundry moulds and cores is known. The major systems which are or have been industrially used are as follows:
(a) The "Carbon Dioxide Process" in which CO.sub.2 is passed through a mixture of sand and sodium silicate. However, the resultant cores or moulds are very sensitive to water and lose strength "damp back" on storage, will not accept aqueous washes and show very poor breakdown on casting. It is necessary to add breakdown agents such as sucrose to promote better breakdown. Over-gassing produces very poor strengths. PA1 (b) The "Sulphur Dioxide Process" disclosed by SAPIC in British Pat. No. 1,411,975 which uses (1) a peroxide which is dangerous to store and dispense, particularly in a foundry environment, and (2) pungent SO.sub.2 which has a low Threshold Limit Value (TLV) and is unpleasant to handle. PA1 (c) The "Isocure" process disclosed by Ashland in British Pat. No. 1,190,644 which uses a benzilic ether phenolic polyol and methylene diphenyl diisocyanate. The reaction between the polyol and diisocyanate is accelerated by gassing with triethylamine or dimethyl ethylamine. The diisocyanates have very low TLV's and react with water preferentially over the polyol so that it is necessary to use dry sand and dry air to convey the sand/binder mix into core box or mould. The amines have relatively low TLV's and their toxicology is not well understood. Cured cores tend to absorb water and lose some strength on storage. Certain casting defects are observed with "Isocure" cores/moulds, e.g. "pinholing" caused by the nitrogen content of the binder which reduces to ammonia in the casting environment, dissolves in the molten metal, and is evolved as small blow holes on cooling; "graphitic defect" which is a deposit of graphite carbon which collects in flakes on the surface of the casting; and "finning" or "veining" caused by the mould or core cracking under the expansion stresses during casting and molten metal running into the cracks. PA1 (a) a weight average molecular weight (M.sub.w) of from 600 to 1500; PA1 (b) a formaldehyde: phenol molar ratio of from 1.2:1 to 2.6:1; and PA1 (c) a KOH: phenol molar ratio of from 0.2:1 to 1.2:1, the binder including from 0.05 to 3% by weight based on the weight of the resin solution of at least one silane, forming the mixture in a vented core or mould box and gassing the formed mixture with at least one C.sub.1 to C.sub.3 alkyl formate to cure the binder.
The present invention enables the rapid and efficient production of foundry moulds and cores without the disadvantages of the prior art as described above.